FIGS. 11(a) to 11(c) are diagrams showing typical microwave waveguides for transmitting microwave signals, in which FIG. 11(a) shows a coaxial line, FIG. 11(b) shows a coplanar line, and FIG. 11(c) shows a rectangular waveguide.
In FIG. 11(a), the coaxial line 150 comprises a signal conductor 111, a grounding conductor 114, and a dielectric 113, such as teflon, filling the space between the signal conductor and the grounding conductor. This coaxial line has a good shielding property and mechanical flexibility. Microwave signals in a frequency band ranging from DC to 60 GHz are transmitted by the coaxial line. For example, in a coaxial line for transmitting microwave signals at 40 GHz, the signal conductor 111 has a diameter of 2 to 3 mm and the grounding conductor 114 has an inside diameter of about 5 mm.
In FIG. 11(b), the coplanar line 151 includes a signal conductor 121 disposed on a dielectric substrate 123 and two grounding conductors 122 disposed on opposite sides of the signal conductor 121. In this structure, since the signal conductor 121 and the grounding conductors 122 are in the same plane, probing from above is easily carried out. In addition, since the degree of freedom in employing frequencies is high, it is widely used in ICs operating at high frequencies and high frequency wafer probes for measuring characteristics of the high frequency band ICs. The coplanar line transmits microwave signals in a frequency band ranging from DC to 150 GHz.
In FIG. 11(c), the waveguide 152 comprises a rectangular grounding conductor 134 filled with air. Since the waveguide 152 has neither a core conductor nor a dielectric which cause an increase in loss, attenuation of high-frequency waves is smaller than in the coaxial line. However, the available frequency range is only 30-50% of design frequency range and, particularly, the DC signal cannot be transmitted. The grounding conductor 134 usually comprises Zn or Cu, and a good conductor, such as Au, is plated onto the internal wall to a thickness of 2 to 3 microns to reduce the loss. This waveguide transmits microwave signals at a frequency band ranging from 40 to 300 GHz. In a waveguide for transmitting microwave signals in 40 GHz the cross-sectional area of the rectangular grounding conductor 134 is 4.about.5 mm.times.2.about.3 mm. The higher the frequency transmitted is, the smaller the sectional area of the waveguide becomes. In a waveguide for transmitting microwave signals at 100 GHz, the cross-sectional area is about 1.2 mm.times.0.5 mm.
FIG. 18 is a schematic diagram illustrating an apparatus for measuring characteristics of a high frequency band IC. In the figure, reference numeral 201 designates a measuring apparatus body and numeral 204 designates an IC chip to be measured. A first probe 203a and a second probe 203b are connected to the measuring apparatus 201 by a first connecting cable 202a and a second connecting cable 202b, respectively. In the measurement, the first probe 203a is applied to a signal input terminal 205a of the IC chip and the second probe 203b is applied to a signal output terminal 205a of the IC chip.
A coaxial line (DC.about.60 GHz) or a waveguide (40.about.300 GHz) are generally used as the cables 202a and 202b connecting the high frequency wafer probes to the measuring apparatus and a coplanar line structure is generally employed for the input or output interface part of the high frequency band IC. Therefore, the coaxial line or the waveguide must be converted to the coplanar line in the probe.
FIG. 12 is an exploded perspective view illustrating a coaxial type high frequency wafer probe in which a coaxial cable is converted to a coplanar line. In FIG. 12, reference numeral 105 designates a probe cover, numeral 106 designates a dielectric blade with a coplanar line structure on its rear surface, and numeral 107 designates a probe body comprising a conductor. A socket 112 comprising a conductor is connected to the probe body 107. An end of a coaxial cable 150 is inserted into the socket 112 and fixed by a nut 115. A tip of the signal conductor 111 of the coaxial cable 150 protrudes to a region where the dielectric blade 106 is to be disposed.
FIG. 13 shows the under surface of the dielectric blade 106. The dielectric blade 106 comprises a dielectric plate 103, a signal conductor 101, grounding conductors 102, a signal contact part 109 disposed on the end of the signal conductor 101, and grounding contact parts 110 disposed on the ends of the grounding conductors 102.
FIG. 14 is a sectional view of the probe of FIG. 12, illustrating the coaxial cable to coplanar line (dielectric blade) transition part. In FIG. 14, the core conductor 111 of the coaxial cable is connected to the signal conductor 101 of the coplanar line. The grounding conductor 114 of the coaxial cable is in contact with the socket 112 fixed to the probe body 107, and the grounding conductors 102 (not shown in FIG. 14) of the coplanar line are in contact with the probe body 107, whereby the grounding conductor 114 of the coaxial cable is electrically connected to the grounding conductors 102 of the coplanar line. In addition, reference numeral 116 designates a conductive member adhered to the coaxial cable 150, through which the coaxial cable 150 is fixed to the probe body 107 by the socket 112 and the nut 115.
FIG. 15(a) is a side view of a waveguide type high frequency wafer probe in which an waveguide is converted to a coplanar line. FIG. 15(b) is an exploded perspective view of the waveguide to coplanar line transition part included in the probe of FIG. 15(a). In FIG. 15(b), a dielectric blade 106 is clamped between upper and lower halves 154a and 154b of the grounding conductor of the waveguide 152. A ridge 118 is disposed on the internal upper wall of the upper grounding conductor 154a. The height of the ridge increases stepwise as it approaches the front end of the grounding conductor, and a tip of the ridge 18 is in contact with a signal conductor of the dielectric blade 106.
A method of connecting the coplanar line (dielectric blade) to the waveguide is illustrated in FIGS. 16(a) and 16(b). FIG. 16(b) is a cross section taken along line XVIb--XVIb of FIG. 16(a). As shown in FIG. 16(b), the end portion of the ridge 118 is connected to the signal conductor 101 of the coplanar line by a lead wire 155.
FIG. 17 is a perspective view illustrating characteristic measurement of a high frequency band IC using either of the conventional high frequency wafer probes shown in FIG. 12 and FIGS. 15(a)-15(b). In FIG. 17, an input or output interface part of a high frequency band IC 160 has a coplanar line structure including a signal electrode pad 162 disposed on an end of a signal conductor disposed on a semiconductor substrate 161 and grounding electrode pads 163 connected to a rear grounding conductor 170 disposed on the rear surface of the substrate 161 through via-holes 164. On the other hand, the conventional probe has the dielectric blade 116, i.e., the downward-looking coplanar line, at the end thereof, and the signal contact part 109 and the grounding contact parts 110 are applied to the signal electrode pad 162 and the grounding electrode pads 163 of the IC 160, respectively.
The application of the high frequency wafer probe to the input or output interface part of the high frequency band IC shown in FIG. 17 is identical to the application of the first or second high frequency wafer probe to the input or output terminal of the high frequency band IC shown in FIG. 18. When the first and second probes 203a and 203b are in contact with the input and output terminals 205a and 205b of the IC chip 204, respectively, millimeter wave signals at about 100 GHz are applied to the input terminal 205a from the first probe 203a, and microwave signals output to the output terminal 205b are taken out by the second probe 203b. The microwave signals are sent to the measuring apparatus 201 through the cable 202b, where the characteristics of the IC chip are measured.
Since the conventional probes shown in FIG. 12 and FIGS. 15(a)-15(b) have a two-stage transition, i.e., a first transition from the coaxial cable to the dielectric blade (coplanar line) and a second transition from the dielectric blade to the interface part (coplanar line) of the IC, the transmission losses and reflection losses at the transition parts unfavorably increase. A loss caused by the transition from the dielectric blade to the interface part of the IC is as small as 0.2 dB. However, about 0.4 dB loss occurs in a transition from a 20.about.30 GHz band coaxial cable to the dielectric blade of a coplanar line, and about 0.8 dB loss occurs in a transition from a 100 GHz band waveguide to the dielectric blade.
FIGS. 19(a) and 19(b) are diagrams illustrating a high frequency wafer probe with a direct transition from a waveguide to a coplanar line of an IC chip, disclosed in Japanese Published Patent Application No. 4-100711, in which FIG. 19(a) is a perspective view thereof and FIG. 19(b) is a cross section taken along line XIXb--XIXb of FIG. 19(a).
This high frequency wafer probe 320 comprises a horizontal waveguide 320a and a vertical waveguide 320b united with the horizontal waveguide 320a. Ground contact parts 323, which can be connected to grounding electrode pads on an input or output interface part of a high frequency band IC chip (not shown), are disposed on the under surface 320d of the waveguide 320a. An aperture 320c is located between the grounding contact parts 323. A conductive ridge 322 is disposed on the internal upper wall of the horizontal waveguide 320a, and the height of the ridge increases stepwise as it approaches the front end of the waveguide 320a. A signal contact part 324, which can be connected to a signal electrode pad of the input or output interface part of the high frequency signal IC chip, is disposed on the tip of the ridge 322 and protrudes through the aperture 320c.
Since the high frequency wafer probe 320 thus constituted includes neither a waveguide to coplanar line transition part nor a dielectric blade of a coplanar line structure, loss of signal strength in the millimeter wave band is reduced, resulting in a characteristic measurement with high precision.
In the characteristic measurement of a high frequency band IC using the high frequency wafer probe 320 shown in FIGS. 19(a) and 19(b), however, the contact parts 323 and 324 of the probe are hidden from view by the horizontal waveguide 320a as shown in FIG. 20(a), resulting in difficulty in positioning the contact parts of the probe accurately on the electrode pads of the IC chip as well as in confirming the contact condition between the contact parts and the electrode pads. Particularly when the electrode pads of the IC chip have different heights, for example, when the grounding electrode pads 163 are higher than the signal electrode pad 162 as shown in FIG. 20(b), an imperfect or faulty contact occurs between the signal contact part 324 and the signal electrode pad 162, which is hard to find in the conventional structure. In addition, since the front end of the horizontal waveguide 320a is covered with the grounding conductor, the probe must be designed considering reflections of high-frequency components at the front end.